1. Field of the Invention
The present invention relates to an inertial unit for mounting on a vehicle. The vehicle may be a land vehicle, an aircraft, or a ship, for which navigation requires knowledge of the position, the speed, and the attitude (heading, roll, and pitching) of the vehicle. The invention also relates to an angle-measurement method.
2. Brief Description of the Related Art
A modern inertial navigation unit generally has an inertial core. The inertial core has inertial sensors such as gyros and accelerometers that are arranged on the axes of a measurement reference frame. The gyros measure angular rotations of the measurement reference frame relative to a geographical reference frame and provide the attitude of the vehicle in the geographical reference frame. The accelerometers measure accelerations that are projected onto the geographical reference frame and then integrated a first time to provide the speed of the vehicle, and then a second time to provide its position. The accuracy of an inertial navigation unit depends directly on the errors of the inertial sensors, and more precisely on the projections of these errors onto the local geographical reference frame; and when performing inertial navigation over a long duration, the position errors depend for the most part on the accuracy of the gyros. The accuracy of the gyros is affected by drift errors (shifting of the measurement origin, such that a zero magnitude is then not measured at zero), by scale factors (scale factor error is an error concerning the coefficient by which a measurement should be multiplied), and by the positioning of the axes (orthogonality error between the measurement axes).
In inertial cores, it is known in particular to make use of vibrating-resonator gyros. Vibrating gyros are axisymmetric Coriolis vibratory gyroscopes (CVG), e.g. having a hemispherical resonator (i.e. a hemispherical resonator gyroscope (HRG)), and they are more generally said to be “type I” as in the document “Type I and type II micromachined vibratory gyroscopes” by Andrei M. Shkel, pp. 586-593, IEEE/ION (Institute of Electrical and Electronics Engineers/Institute of Navigation) PLANS 2006, San Diego, Calif., USA. Such gyros operate either in an open loop, in which ease they serve to measure an absolute angle of rotation on the basis of measuring an angle representing the position of the vibration of the resonator relative to measurement electrodes (electric angle), or else in a closed loop, in which case they serve to measure a speed of rotation on the basis of a measurement of the electric current needed to maintain the electric angle at a given value. The measurements provided by such vibrating gyros may be affected by errors that are essentially a function of the position of the vibration relative to the measurement electrodes. These errors are thus variable as a function of the position of the vibration as represented by the electric angle.
In order to improve the accuracy of such inertial units, it is common practice to use gyros presenting the best possible performance. Nevertheless, such gyros are relatively expensive.